The project is using free Godot game engine v3.5
The rendered nodes are taken from /output/data.json file (which is deployed on GitHub Pages directly and loaded over http at the init with dynamic node creation & rendering)
If you want to substitute the data with your own, for now your only option is to download the repo, install Godot and run it locally after changing the file. Beware : running in debug mode loads the file from user folder, so you need to put the file there (In Godot click Project>Open user data folder)
I might add the feature&UI to be able to (re)load data.json from any source live (if there is demand)